This invention relates to a lubricant employed for assuring running durability of a magnetic recording medium and a magnetic recording medium having the lubricant held on its magnetic layer.
In general, a magnetic recording medium, especially a magnetic tape, is run at an elevated velocity in sliding contact with a magnetic head in the course of writing/readout of magnetic signals. Such running of the magnetic tape must be made smoothly and under a stabilized condition. Since the magnetic layer of the magnetic recording medium has its surface smoothed for effecting high-density recording, it is increased in its effective contact area with sliding components, such as a magnetic head or a guide roll, during sliding, so that the phenomenon of agglutination or sticking tends to be produced. In particular, in the case of a thin magnetic metal film type magnetic recording medium, in which a magnetic metal material is deposited by evaporation on a non-magnetic substrate so as to be used as a magnetic layer, the above-mentioned problem related with rise in the frictional coefficient is liable to occur because of the extremely smooth surface of the magnetic layer.
For example, in the case of an 8-mm vide deck, a magnetic tape is placed around a drum along ten or more guide pins and is run by a pinch roll and a capstan under a tape tension of about 20 g and at a running velocity of 0.5 cm/second. Since the magnetic layer of the magnetic tape is contacted with stainless steel guide pins, the magnetic tape undergoes stick slip to produce a hissing sound with increase in the frictional coefficient between the magnetic tape and the guide pin. If the hissing sound is produced from the running tape during reproduction, a "cramped" playback picture is produced.
In the paused state, the magnetic head is run at an elevated velocity through the same area of the magnetic tape, so that, if the frictional coefficient is increased, the magnetic layer is worn out to lower the playback output. In particular, with the thin magnetic metal type magnetic tape, in which the magnetic layer is of an extremely thin thickness, such wear of the magnetic tape raises a serious problem.
If the magnetic head is to have a contact start step (CSS) with respect to the magnetic disc, as in the case of a hard disc device, the problem related with rise in the frictional coefficient is produced. The reason is that the magnetic head is slid in contact with the magnetic disc during floating and landing of the magnetic head. If, in such case, the frictional coefficient is increased, not only the problem of the wear to the magnetic layer of the magnetic disc but also the problem of head crushing is incurred. For assuring reliability of the product quality, the frictional coefficient after 20,000 times of CSS operations is desirably 0.5 or less.
For solving the problems caused by rise in the frictional coefficient and the problem of running durability as mentioned above, it is contemplated to use a variety of lubricants. For this reason, attempts have been made for top-coating the lubricant on the magnetic layer or incorporating the lubricant in a magnetic coating in the case of a coated type magnetic recording medium.
It is required of the lubricant to be (i) superior in low-temperature properties for assuring pro-set lubricating effects for use in frigid districts, (ii) coatable to an extremely small thickness to reduce the spacing loss with respect to the magnetic head to as small a value as possible and (iii) usable and be able to maintain lubricating effects for a long time, to say nothing of having optimum lubricating effects with respect to the magnetic head.
For accomplishing the above properties with a film thickness of a molecular level, that is on the order of several nanometers, it is necessary to investigate the molecular structure of compounds making up the lubricant. Among the lubricants now in use as lubricants for a magnetic recording medium, there are a silicon-based lubricant, a hydrocarbon based lubricant and a lubricant composed of fluorine compounds.
The lubricant composed of a silicon-based compound are used for a coated type magnetic recording medium because the lubricant is superior in thermal stability and low in vapor pressure. However, if the lubricant is applied to a thin magnetic metal film type magnetic recording medium, sufficient lubricating effects cannot be produced, such that the design statements for durability are not met on accelerated abrasion tests for pin-on discs or CSS tests.
The lubricant composed of a hydrocarbon compound is prevalently employed for a coated type magnetic recording medium. However, it is inferior to the lubricant composed of a fluorine compound in thermal or chemical stability. On the other hand, since the frictional polymer is yielded due to reaction produced by friction, the lubricating effects are lowered, while fatal troubles may also be incurred. If used for a thin magnetic metal film type magnetic recording medium, the lubricant composed of the hydrocarbon compound exhibits superior lubricating effects. However, the lubricant can hardly be put to practical use because of high vapor pressure.
On the other hand, the lubricant composed of a fluorine compound is now employed most extensively. In particular, the lubricant containing perfluoro polyether is employed widely because it is excellent in lubricating effects and surface protective effects. The reason is that perfluoro polyether is lower in viscosity than other compounds having a comparable molecular weight, because the ether linkage CF.sub.2 --O--CF.sub.2 is flexible, and that the viscosity is not changed over a wider temperature range. Chemical inactivity, low vapor pressure and excellent water-repellency of the lubricant may also be recited as contributing factors.
The properties of perfluoro polyether depend appreciably on the repetitive units of the main chain. For example, perfluoro polyether per se is chemically unstable so that it is insufficient in absorbent power with respect to the surface of the magnetic layer. However, if polar groups, such as hydroxyl groups or piperonyl groups, are introduced as terminal groups, the absorbent power is improved, thus increasing durability of the magnetic recording medium.
Although the lubricant composed of a perfluoro polyether compound has superior characteristics as lubricant for a magnetic recording medium, it is open to doubt whether the lubricant can cope with the problem of high heat of friction concomitant with the progressively increasing processing speed of the magnetic recording system. Although the reliable method for measuring the heat of friction has not been established to date, it is estimated that the heat of friction reaches several hundred degrees centigrade in case the relative velocity between the magnetic recording medium and the magnetic head exceeds several meters per second.
Although perfluoro polyether is stable in air even at a temperature higher than 350.degree. C., it is estimated that the reaction of decomposition is promoted at the above-mentioned elevated temperatures in the presence of metal alloys, such as iron or titanium alloys, Louis acids, such as AlCl.sub.3, FeF or Al.sub.2 O.sub.3 or Louis base. If the reaction of decomposition occurs, the lubricating effect is deteriorated, with the result that the operational reliability of the magnetic recording system is lowered.
On the other hand, perfluoro polyether suffers from the problem that it is not soluble in general-purpose solvents such that a so-called Freon based solvents need to be employed.